The present invention relates to a method of manufacturing a glazing panel comprising sheets which are joined together along the margin of the panel using heat-activatable bonding medium which is electrically conductive and/or in contact with electrically conductive material comprised within the panel and which is activated in situ by induction heating.
Such a method is applicable for example in the manufacture of hollow glazing panels, the sheets being bonded together by intervening spacing means. The spacing means may for example comprise a metal spacer rail or rails which is or are bonded to metallised margins of the sheets by solder which is melted in situ. As an alternative a heat-activatable adhesive composition can be used for bonding the sheets to a spacer of metal, glass or other material. As a further alternative the spacing means may be constituted by the heat-activatable bonding material itself.
Bonding by induction heating is known per se in several fields. For example British Patent Specification No. 1 167 193 is mainly concerned with plastics packaging. In that specification the parts to be joined are pressed between an inductive loop and a non-magnetic electrically conductive reinforcing member which serves to concentrate the magnetic field produced by the loop in the region of conductive material distributed along the joint line to heat it and soften the adjacent plastics materials so that the plastics parts become fused or sealed together. There is passing reference to use of the invention with cardboard stock, wood, rubber, glass and paper in addition to plastics material, though no detail of any such use is given. In any event the process and apparatus of British Patent Specification No. 1 167 193 would not be suitable for series production of glazing panels, and in particular of hollow glass panels, in view of the repeated contact of the apparatus with a panel.
As evidenced by the prior art, attempts to use induction heating in the manufacture of glazing panels have followed a rather different course. However most of these prior proposals are of a general nature in the sense that they refer to induction heating as one of the possible ways in which jointing material can be heated in situ, but give at best very little information concerning the form of induction heating apparatus and the procedures which should be used. Among various proposals to join assembled components of a hollow glazing panel by using an induction heating step are those described in British Patent Specifications Nos. 831 166, 1 307 843 and 1 506 282.
In the above mentioned patent specifications: British Pat. No. 831 166 simply states that the assembled components, in that case glass panes and an intervening copper spacer strip, can be placed on a conveyor, moved into a tunnel oven wherein the work assembly is raised to 500.degree. C. and then moved past an alternating magnetic field whereby the temperature of the spacer strip is raised by the induced current sufficiently to fuse the edges of the ring to the glass panes. In this method the heating is sufficient to melt the portions of glass which are in contact with the metal ring so that no separate bonding medium is needed, but the specification does indicate that the metal can be coated with a layer of a bonding agent such as easy-melting powdered glass or borax, in order to improve the wetting of the metal by the molten glass.
British patent specification No. 1 307 843 states that bonding medium for bonding the glass panels of a double glazing panel to an intervening metal spacer can be activated in situ by subjecting the assembly to an electrical heating treatment such as induction or resistance heating; but it does not give any information concerning suitable electrical heating apparatus or procedures.
British Patent specification No. 1 506 282, which likewise refers to heating of the spacer rail or rails of a double glazing panel by means of an inductive eddy current, does include an outline of possible procedures. The specification says that the spacer rail or rails can be heated as a whole by means of inductive eddy currents and goes on to state that satisfactory results may be achieved in many cases if a relatively large portion of the spacer rail is gradually heated by means of induced eddy currents to the temprature necessary for the joint sealing and the heat is thereafter allowed to progress successively and gradually along the spacer rail, e.g. by a slow successive relative displacement of the eddy current source with respect to the spacer rail in the longitudinal direction. In a specific embodiment use is made of high-frequency coils and a longitudinal portion of the spacer rail corresponding substantially to the diameter of the high-frequency field is slowly heated to the jointing temperature before the panel assembly is displaced to conduct its adjacent edge areas successively through such field.
When assessing the suitability of an inductive heating method for use in the production of panel joints under industrial mass production conditions, various factors need to be considered. Most important of course is the quality of the panel joints and the reliability with which a given joint standard can be reproduced. The panel joints must not only have a certain minimum strength to withstand forces imposed on the panel in use, but they should be of uniform quality around the panel. Furthermore, in series production of panels, the standard of the joints formed must be consistent from panel to panel. Other important factors are the time required for making the joints of each panel and the facility with which the work handling operations at the heating station can be accomplished and mechanised if required. Yet another important factor is the cost of the energy consumption involved in generating the necessary heat.